Specifying method for deserializing enum variants

Currently, basically any standard enum deserialization will go through `deserialize_identifier()`, which in turn goes through `visit_str()`. But most implementations will want to support `visit_u64()` and `visit_bytes()`, and will likely want to test this behavior as well. When building a `Deserializer`, we should allow users to specify what method will be used by default.

I am still unsure of how exactly this will look. Should the user still pass in a `Token::UnitVariant` or similar? Or should we just allow other token types in this position? Should we only allow `u64` and `bytes` as alternatives, or should we allow all kinds of things here?